1. Field of the Invention
The present invention relates to a method of mounting an electronic component on a substrate by bonding together electrode terminals of the substrate and the electronic component, wherein the electrode terminals of at least one of the substrate and the electronic component are composed of solder bumps, and to a mounting apparatus.
2. Related Art
Techniques where a semiconductor chip is flip-chip bonded to a substrate by bonding solder bumps, as the electrode terminals of a semiconductor chip as an electronic component, to pads, as the electrode terminals of the substrate, are in conventional use.
A typical method will now be described with reference to FIGS. 5A to 5F. Note that in FIGS. 5A to 5F, only one pair out of the large number of electrode terminals formed on the semiconductor chip and on the substrate are shown.
FIG. 5A shows a substrate 12 on which a semiconductor chip 10 will be mounted. Reference numeral 14 designates a pad as an electrode terminal formed on the surface of the substrate 12, and reference numeral 16 designates a solder resist that covers the surface of the substrate 12. FIG. 5B shows a state where flux 18 has been expelled from a nozzle so that the surface of the pad 14 is covered by the flux 18. FIG. 5C shows a state where a solder bump 20 as an electrode terminal of the semiconductor chip 10 has been aligned with the pad 14 and the semiconductor chip 10 has been provisionally fixed to the substrate 12 using the viscosity of the flux 18.
FIG. 5D shows a state where the semiconductor chip 10 has been bonded to the substrate 12 by reflowing the solder bump 20. When the solder is reflowed, an oxide film is removed due to the activating effect of the flux 18, so that the solder bump 20 is tightly bonded to the pad 14. FIG. 5E shows a state where the flux 18 remaining in the periphery of the pad 14 has been removed by washing. The flux 18 includes a component that corrodes the electrode and the like. Accordingly, it is necessary to remove the flux 18 remaining on the substrate 12 by washing. FIG. 5F shows a state where the gap between the substrate 12 and the semiconductor chip 10 has been filled with underfill resin 22 to finally mount the semiconductor chip 10 on the substrate 12.
With the conventional method shown in FIGS. 5A to 5F, the flux 18 is applied so as to improve the bonding of the solder bump 20 and the pad 14 by reducing a film of oxidized solder formed on the outer surface of the solder bump 20.
With the conventional method described above, it is necessary to wash away the flux, which leads to increased complexity and cost for the manufacturing process. For this reason, Patent Document 1 discloses a method for removing the oxidized solder film on the surface of the solder bump without using flux. In a state where the solder bumps of the semiconductor chip contact the pads of the substrate, ultrasonic vibration is applied to the semiconductor chip before the solder bumps are reflowed to cause the solder bumps to rub against the pads and thereby remove the oxidized solder film on the surface of the solder bumps. By doing so, it is possible to improve the bonding between the solder bumps and the pads after reflowing (see Paragraph 0011 of Patent Document 1).
Patent Document 2 also discloses a technique where ultrasonic vibration is applied to remove an oxide film on the solder bumps and an oxide film on the metal pads (see Paragraph 0019 of Patent Document 2). Patent Document 2 discloses that the atmosphere used during flip-chip bonding is the normal atmosphere, an inert atmosphere such as nitrogen or argon, or a reducing atmosphere such as carbon dioxide gas or hydrogen (see Paragraph 0023 of Patent Document 2).
The technique disclosed in Patent Document 3 is another conventional method where it is not necessary to wash away flux. The technique disclosed in Patent Document 3 will now be described with reference to FIGS. 6A to 6C.
FIG. 6A shows a state where flux fill 30 has been expelled from a nozzle 26 onto a surface of the substrate 12 on which the pads 14 are formed so that regions where the pads 14 are formed are covered with the flux fill 30. FIG. 6B shows a state where the solder bumps 20 are pressed against and bonded to the pads 14 while applying ultrasonic vibration to the semiconductor chip 10. If the solder bumps 20 are pressed against the pads 14 while applying ultrasonic vibration to the semiconductor chip 10, the filler inside the flux fill 30 will be pressed away from the surfaces of the pads 14 by the solder bumps 20, so that the solder bumps 20 will come into contact with the pads 14. Since the flux fill 30 acts as flux, an oxidized film that covers the pads and the like is removed by the energy of the ultrasonic vibration, and it is possible to bond the solder bumps 20 to the pads 14 using only the energy of the ultrasonic vibration (see Paragraphs 0014, 0015 of Patent Document 3).
FIG. 6C shows a state where the solder bumps 20 have been bonded to the pads 14 to mount the semiconductor chip 10 on the substrate 12. The solder bumps 20 are bonded to the pads 14 and the gap between the semiconductor chip 10 and the substrate 12 is underfilled with the flux fill 30 (see Paragraph 0016 of Patent Document 3).
Patent Document 1
Japanese Laid-Open Patent Publication No. 2000-174059 (Paragraph 0011)
Patent Document 2
Japanese Laid-Open Patent Publication No. 7-115109 (Paragraphs 0019, 0023)
Patent Document 3
Japanese Laid-Open Patent Publication No. 2005-26579 (Paragraphs 0014 to 0016, FIG. 1)